1. Field of the Invention
The present invention relates to a method and apparatus for adaptive filtering of a signal having a desired component and an undesired component, and more specifically, to reducing the level of an undesired noise component in a physiological signal by adaptive filtering of the physiological signal using a synthetic reference signal which is modeled to exhibit a correlation with the desired signal.
2. Description of the Prior Art
The measurement of various types of physiological signals is generally a difficult task because the underlying physiological processes that generate physiological signals generate such signals at very low amplitude levels. Additionally, during acquisition of the physiological signals, the physiological processes, and/or sensors associated therewith, typically also generate or become the sources for a noise component that becomes an undesired part of the desired physiological signals.
For example, when electrocardiogram (ECG) signals of a patient are measured, sensors detect not only the electrical activity generated by the electrical depolarization of the heart, a relatively weak signal by the time it reaches the skin of the patient, but also electrical signals, artifacts, generated by the activity of other muscles in the patient. Furthermore, external electrical interference, such as the 60 Hz line frequency signals and electrical signals emanating from nearby electrical equipment are also acquired as noise components of the desired physiological signal. Hereinafter, such noise and/or artifact signals are also referred to as the noise, artifact, or undesired component of the physiological signal.
Another common physiological signal measurement suffering from low levels of desired signal and relatively higher levels of the undesired component is the measurement of the blood oxygen saturation level of a patient using a pulse oximeter. As well known by those of ordinary skill in the art, a pulse oximeter measures arterial blood oxygen saturation using a sensor arrangement containing two LED's and a photodiode detector. The sensor is applied directly to a well perfused part of a patient, such as at a finger or ear. Each LED of the sensor transmits radiation at a different one of two wavelengths, typically one being red and the other being infrared, to the patient. The photodiode detector is responsive to the red and infrared light for developing red and infrared electrical signals that are affected, via transmission or reflection, by the patient's blood flow in the area between the two LED's and the light receiving portion of the photodiode detector. The greater the oxygenation of the blood, the less of the emitted red light is detected, due to greater absorption of the red light by the patient's blood. In pulse oximeters, the acquired red and infrared signals are processed to develop a measurement indicative of the current blood oxygenation level of the patient. Additionally the acquired light signals can be processed further to develop a measurement of the pulse rate of the patient, since, as well known, the pulsatile component of the light signals results mainly from ventricular contractions of the heart.
Processing of the red and infrared signals for determining blood oxygenation is based on the Beer-Lambert law, as well known, wherein a ratio is generally used to compare the AC and DC components of the red light (AC.sub.r and DC.sub.r, respectively), to the AC and DC components of the infrared light (AC.sub.ir and DC.sub.ir, respectively), in accordance with the following equation: ##EQU1## The resultant value is applied to an experimentally-determined reference table (look-up-table) to provide the final determination of the measured level of the blood oxygenation. Additionally, as noted above, the AC components of the signals can be further processed to generate an estimate of the pulse rate.
The blood oxygenation and pulse rate measurements made from optically acquired signals are highly prone to inaccuracies due to the undesired noise and/or artifact components of the acquired signal. The noise components typically result from electrical interference (lights, electro-surgical and other electrical equipment being operated near the patient), and artifacts typically result from patient movement (causing a relative movement, and concomitant change in light path, between the LED's and detector of the sensor, or even worse, the sudden admission of room light into the receiving area of the photodiode detector). Furthermore, the AC component of the acquired signals (which result from the pulsatile characteristic of the blood), is very small, typically on the order of only 1%-5% of the DC value of the acquired signals, as is also typical of physiological signals. Consequently, such noise and artifacts are extremely detrimental to accurate pulse oximetry measurements, leading to the serious problem of an incorrect assessment of the patient's condition, as well as false alarms to the user of the oximeter.
U.S. Pat. No. 4,955,379 entitled MOTION ARTIFACT REJECTION SYSTEM FOR PULSE OXIMETERS, issued Sep. 11, 1990, discloses a band-pass filtering (BPF) technique for removing noise artifacts from pulse oximetry signals. More specifically, the AC components of each of the acquired red and infrared signals is initially filtered by a BPF that is broadly tuned to the expected heart rate frequency. The output of the BPF is applied to a frequency determining circuit, whose output is then used to cause the BPF to track the frequency determined by the frequency determining circuit. The theory of this technique is that most of the energy (and information) in the AC signal is contained at the fundamental frequency, and since the fundamental frequency should be the pulse rate, the frequency determining circuit will determine the pulse rate as the fundamental frequency and control the BPF to exclude all other frequencies, along with artifacts. Unfortunately, it is quite possible that the fundamental frequency determined by the frequency determining circuit may in fact be a noise signal, such as one that is generated by electrical equipment, causing the oximeter to process the signal and report erroneous information. Furthermore, even if the fundamental frequency of the pulse rate is correctly determined, since other frequency components of the desired pulse signal are excluded, a degraded performance of the oximeter can result. Consequently, this technique is undesirable.
U.S. Pat. No. 4,928,692 entitled METHOD AND APPARATUS FOR DETECTING OPTICAL PULSES, issued May, 29, 1990, discloses a technique wherein the R-wave portion of a patient's ECG waveform is correlated in time with the optical signals acquired by a pulse oximeter. The correlation is used to develop an enabling signal for processing of the acquired optical signals by the oximeter. The theory is that since the pulsatile component of the optical signals contain the information, and the occurrence of the pulses can be predicted to follow an ECG R-wave by a certain amount, selective timing of oximeter enablement will prevent artifact from being admitted into the oximeter and erroneously processed. Unfortunately, since artifacts can occur at any time, and in general are not in any way correlated so as to have any relation to occurrence of an ECG R-wave, this technique is also undesirable.
U.S. Pat. No. 5,482,036 entitled SIGNAL PROCESSING APPARATUS AND METHOD, issued Jan., 9, 1996 is representative of a technique that uses an adaptive noise cancellation filter for reducing noise in pulse oximetry signals acquired using a sensor arrangement having two light sources. FIG. 5 of this prior U.S. Pat. No. 5,482,036, illustrates the application of linear adaptive noise cancellation to pulse oximetry. The acquired signal S.sub..lambda..alpha. comprises two components: a desired signal component Y.sub..lambda..alpha. (a modulation signal that would be obtained from a pulse oximeter under ideal conditions), that is additively combined with a noise signal component n.sub..lambda..alpha.. A reference signal n' that has a significant similarity to the noise component is provided. The objective of the cancellation filter is to transform the reference signal n' into a signal b.sub..lambda..alpha. having as close an approximation of the noise component n.sub..lambda..alpha. as possible. Then, by subtracting the noise component approximation from the contaminated signal, a reconstruction Y'.sub..lambda..alpha. of the uncontaminated component of the input signal is obtained. Conversely, a reference signal n' that has a significant similarity to the desired signal Y.sub..lambda..alpha. can be provided, and an approximation Y'.sub..lambda..alpha. built up for Y.sub..lambda..alpha. using operations on n'. Built up signal Y'.sub..lambda..alpha. can then be used as the output of the cancellation filter. The basic idea is that if the reference signal contains substantial information about only one, not both, of the two input signals, the input signal S.sub..lambda..alpha. can be separated into some approximation of the desired signal component Y.sub..lambda..alpha. and some approximation of the noise signal component n.sub..lambda..alpha..
However, in pulse-oximetry, a reference signal is not readily available. In the forenoted U.S. Pat. No. 5,482,036 a reference signal is generated from the measured lead signals using a technique based on the fact that the desired portions of the acquired red and infrared signals are linearly related at a given level of blood oxygen saturation. More specifically, the acquired red and infrared signals are subtracted from each other after determination of an appropriate scaling factor w, for generating the approximation of the noise component, which approximation is then used as the reference input to the adaptive filter to develop a reconstruction of the desired signal. However, a significant problem with the above technique is that by combining the acquired signals to generate a reference signal that describes the noise component, some part of the desired component s may be included in the reference signal. Consequently, the desired signal s, or a portion thereof, will be erroneously identified as noise, thereby causing significant errors in reconstruction of the desired signal. The same problem exists if the system is operated conversely, where the acquired red and infrared signals are subtracted from each other after determination of a different appropriate scaling factor w, for generating a signal substantially similar to the desired signal component s, which approximation is then used as the reference input to the adaptive filter. In the latter case some part of the noise component n may be included in the reference signal, thereby also causing significant errors in reconstruction of the desired signal.
It would be desirable to provide a more reliable manner of reducing the noise component in an acquired physiological signal.
As will be described next, the present inventor has discovered that in an adaptive cancellation arrangement, to reduce noise and other artifacts from a desired component of a noisy signal, when the basic structure of the desired signal is known, knowledge of the basic structure can be used to improve the operation of the adaptive cancellation arrangement.